1. Field of the Invention
The present invention relates to a projection image display device, more particularly to a projection one-panel color liquid crystal display device providing color display with one liquid crystal display panel without using any color filters. Such a display device can be used particularly for a compact projection color liquid crystal television system or information display system.
2. Description of Related Art
The projection liquid crystal display device must be provided with a light source since it does not emit light itself. The projection liquid crystal display device has great advantages over projection Braun-tube display devices, because of wider variety of reproducible colors, smaller size, lighter weight and unnecessary convergence adjustment. Further development of the projection liquid crystal display device is highly expected.
For carrying out projection color display using a liquid crystal display panel, there are two systems which are conventionally employed, that is, a three-panel system using three liquid crystal display panels for three primary colors and a one-panel system using only one panel. The former three-panel system includes an optical system for dividing white light into three primary color lights, i.e. red, green and blue (hereafter referred to as R, G and B), and three liquid crystal display panels for controlling the three color lights to produce an image. The image is displayed in full color by optically superposing images of all the color lights. The three-panel system utilizes light emitted by a white light source effectively. However, the optical system is complicated and the number of components is large. Therefore, the three-panel system is generally disadvantageous in cost and size, compared with the later-described one-panel system.
The latter one-panel system is designed to project an image on a liquid crystal display panel having a color filter pattern of the three primary colors in mosaic or in stripes by a projection optical system, for example as described in Japanese Unexamined Patent Publication No. Sho 59(1984)-230383. The one-panel system uses only one liquid crystal panel and a optical system less complicated than that of the three-panel system. Therefore the one-panel system is suitable for a low-cost, small-sized projection system. However, because the color filter used in the one-panel system absorbs light, the one-panel system can only provide images one-third as bright as those provided by the three-panel system using a light source of equal brightness. As for resolution, because three pixels of the liquid crystal display panel corresponding to R, G and B are required to operate as a group to display an image, the resolution of the one-panel system is also one-third as good as that of the three-panel system.
Using a brighter light source may be one solution to the poor brightness. The brighter light source, however, consumes more power, and therefore is not desirable for consumers' use. In addition, since luminous energy of light absorbed by the color filter changes to heat, the brighter light source not only causes rise in temperature of the liquid crystal display panel but also accelerates loss of color of the color filter.
Therefore, to utilize provided light effectively is important for heightening the utility value of the projection image display device.
In order to improve the brightness of the one-panel liquid crystal display device, it has been proposed to throw white light on dielectric mirrors such as dichroic mirrors arranged fanwise, divide the white light into three luminous fluxes of R, G and B, direct the luminous fluxes incident at different angles on a microlens array disposed on the light source side of the liquid crystal display panel, and separately sending each of the luminous fluxes passing through the microlenses according to their incident angles onto a site on the liquid crystal which is driven by a display electrode to which a color signal corresponding to the luminous flux directed onto the site is applied, thereby the device making better use of the provided light [see Japanese Unexamined Patent Publication No. Hei 4(1992)-60538].
Also, for better utilization of the light, a device using transmission type hologram elements corresponding to R, G and B in the light from the light source in place of the above-mentioned dielectric mirrors and a device using a transmission type hologram element having periodic mechanism corresponding to a pixel pitch of the liquid crystal display panel, thereby the device being provided with functions of a microlens and a dielectric mirror, are described in Japanese Unexamined Patent Publication Nos. Hei 5(1993)-249318 and Hei 6(1994)-222361 respectively.
In order to solve the poor resolution, i.e., the other deficiency of the one-panel system, a one-panel system using a field sequential system which has resolution equal to the three-panel system is described on pages 115 to 123 of "Color Liquid Crystal Display" (written by Shunsuke Kobayashi published by Sangyotosho on Dec. 14, 1990). The field sequential system takes advantage of a phenomenon in which, when colors are sequentially changed in time division at such a rate that the eye cannot follow, all the colors are perceived as a mixture (this phenomenon being called a continuous additive color mixture). FIG. 29 shows a exemplary structure of the field sequential system.
Referring to FIG. 29, a disc constituted of color filters of R, G and B is rotated at a high rate in accordance with vertical scanning of the liquid crystal display panel, and an image signal corresponding to the color of a filter at each divided time is input to the liquid crystal display panel. A observing person perceives a compound of images corresponding to the colors. Therefore, unlike the above-described one-panel system, since a dot of the liquid crystal display panel displays images of R, G and B at divided times, the resolution thereof equals that of the three-panel system.
Further, another example of the above-described field sequential system is described on pages 249 to 252 of "Euro Display '93," in which white light from a light source is divided into luminous fluxes of R, G and B with dielectric mirrors, the luminous fluxes are directed onto different areas of a liquid crystal display panel, and a cubic prism disposed in front of the liquid crystal panel is rotated so that a position irradiated by each of the luminous fluxes changes.
However, though the systems described in Japanese Unexamined Patent Publication Nos. Hei 4(1992)-60538, Hei 5(1993)-249318 and Hei 6(1994)-222361 have improved the brightness, none of them have improved the resolution. The resolution remains one-third as good as that the three-panel system because three dots corresponding to R, G and B operate as a group to display an image.
The field sequential system has realized the resolution equal to that of the three-panel system, but still has the same problem with brightness as the conventional one-panel method.
Besides, the system described on pages 249 to 252 of "Euro Display '93" requires extremely parallel light to be provided so that the luminous fluxes of R, G and B will not overlap. Due to this requirement about the light, the system cannot make full use of light.
Therefore, none of the above-mentioned systems have solved both the problems of poor brightness and resolution.